In the current specifications of the third generation mobile networks (referred to as UMTS), the system utilises the same well-known architecture that has been used by all main second generation systems. A block diagram of the system architecture of current UMTS network is presented in FIG. 1. The UMTS network architecture includes the core network (CN), the UMTS terrestrial radio access network (UTRAN), and the user equipment (UE). The core network is further connected to the external networks, i.e. Internet, PLMN, PSTN and/or ISDN.
The GSM Phase 1/2 Core Network consists of network switching subsystem (NSS). The NSS further consists of the following functional units: Mobile Switching Center (MSC), Visitor Location Register (VLR), Home Location Register (HLR), Authentication Center (AC) and equipment identity register (EIR). The GSM Phase 2+ enhancements to the GSM phase 1/2 CN are serving GPRS (General Packet Radio Service) support node (SGSN), gateway GPRS support node (GGSN) and CAMEL service environment. The most important new feature that is introduced with GPRS is packet switching (PS) which improves the capacity of the network. For UMTS, only minor modifications to the GSM Phase 2+ core network are needed. For instance, allocation of the transcoder (TC) function for speech compression.
The UTRAN architecture consists of several radio network subsystems (RNS). The RNS is further divided into the radio network controller (RNC) and several base stations (BTS, referred to as B nodes in the 3GPP specifications).
In this architecture there are several different connections between the network elements. The Iu interface connects CN to UTRAN. The Iur interface enables the exchange of signalling information between two RNCs. The signalling protocol across the Iur interface is called. radio network subsystem application part (RNSAP). The RNSAP is terminated at both ends of the Iur interface by an RNC. The Iub interface connects an RNC and a node B. The Iub interface allows the RNC and node B to negotiate about radio resources, for example, to add and delete cells controlled by node B to support communication of dedicated connection between UE and S-RNC, information used to control the broadcast and paging channels, and information to be transported on the broadcast and paging channels. One node B can serve one or multiple cells. UE is connected to node B through the Uu radio interface. UE further consists of a subscriber identity module (USIM) and mobile equipment (ME). They are connected by the Cu interface. Connections to external networks are made through Gateway MSC (towards circuit switched networks) or GGSN (towards packet switched networks).
FIG. 2 further presents current GERAN architecture. GERAN (GSM/EDGE radio access network) is an enhanced GSM radio access network. Enhanced here means that GERAN uses EDGE as a radio technology. EDGE allows use of UMTS services with 800/900/1800/1900 MHz frequency bands. GERAN offers full advantages of GPRS (General Packet Radio System) to be explored. The base station subsystem (BSS) of GERAN is connected to the GSM core network by Gb (between BSS and a GSM SGSN) and A (between a BSS and a GSM MSC) interfaces. BSS is further connected to UMTS network by interfaces Iu-ps (between a BSS and a 3G SGSN) and Iu-cs (between a BSS and a 3G MSC). BSS is further connected to the RNC of UTRAN or to another BSS by Iur-g interface. BSS includes a base station controller (BSC) and base transceiver stations (BTS). In GERAN the air interface between BTS and Mobile Station/user equipment (MS) is called Um. An IP-RAN (Internet Protocol Radio Access Network) is a RAN architecture that is fully optimised to carry IP traffic. In this configuration the division of functionalities between network elements is fundamentally redefined to suit the needs of IP traffic. This is clearly different from just using IP as a transport solution with the existing network architectures like GSM (Global System for Mobile Communications) and CDMA (Code Division Multiple Access) based radio access networks.
IP is currently the most ‘future proof’ technology offering unprecedented economies of scale, and the possibility to integrate right across the industry; fixed, wireless and mobile.
Future networks will contain multiple radio access technologies, such as GSM, BlueTooth, IEEE 802.11a/b, BRAN HL2 and WCDMA. The IP-RAN will provide a common base to tie all these technologies together in a single RAN. This will enable services to be used seamlessly across the RAN.
The benefits of IP, and IP enabled Radio Access Network (IP-RAN), can be seen clearly and they can be summarised as follows. The primary driver for the increased usage of IP is derived from operators' abilities to create new and easily customisable services over the de-facto service creation environment, the Internet. Secondly, as the content is expressed in the Internet Protocol, native support for IP makes networks more optimal for this form of traffic and operational, and capital expenditure savings over the whole network are significant. Thirdly, IP integrates various access and transport technologies and standards, including fixed, wireless and mobile, into common service creation and delivery networks.
In order to obtain the most efficient RAN architecture, which is based on using the good characteristics of IP, some functionality has to be relocated between network elements. In the most revolutionary architecture we no longer have a network element commonly known as a BSC (Base Station Controller) or RNC (Radio Network Controller), although this functionality must remain in the RAN. However, in evolutionary architectures, the RNC and BSC are still used.
Macrodiversity combining, or soft handover is a specific function of CDMA technology. As IP-RAN architecture will enable very large RAN GateWays, the location of the Macrodiversity Combiner can no longer be centralised for all BTSs (Base Transceiver Station) in RAN. Therefore, in one embodiment of the IP RAN there has been chosen to locate some RNC functionality in the BTSs to enable soft handover and associated signalling to happen along the shortest path, producing minimum delay and signalling load to those parts of networks where this is not necessary.
Referring to the above state of art description it can be said that IP RAN is realised by implementing most of the RNC (or BSC) functionality in the BTS (IP BTS). Only Paging, basic O&M (Operation and Maintenance) and configurations, Location Calculation functions and Common Radio Resource Management may be implemented in separate servers outside the BTS site.
In the distributed architecture of IP RAN the number of BSC/RNC that has to be connected to the neighbouring RAN and the core network CN is around hundreds times higher than in the normal case. The consequence is that the CN and RAN/BSS equipment cannot handle this situation, due to restrictions in the implementation of the configurations of neighbour RNC/BSC. For example the handling of the external addressing of the different entities and the use of the identifiers (in 3G networks an identifier is assigned to a UE that has an existing signalling connection to the radio access network) becomes very complicated.
Thus, there is an increasing need for a logical or virtual network entity that can handle the very complicated external addressing of the different network elements as well as the use of temporary identifiers assigned to the user equipment. Also there is an obvious need for a gateway type network element which can hide the distributed radio access network control element, i.e. the distributed functionalities of RNC/BSC from the CN's or neighbouring radio network's point of view.